What percent hydrogen peroxide did you use?
I tried SF nematodes, which worked pretty well. The problem seemed to be worst in my tradescentia zebrina, so I dumped the entire pot and put the plant in new potting mix. After a couple weeks, I sprinkled cinnamon all over the place in every pot and then I sprayed the transcentia zebrina with a mixture of alcohol, mild dish soap (liquid) and water. That seemed to get stop the gnats and it didn't harm the plant. However, some of my other plants ended up being damaged. Two calathea are down to one leaf, so I'm hoping they will hang on til spring when they come out of domancy. The dracaena marginata is surviving, but I lost the smaller one. All the other tropical plants are fine.
That said, I would like to try using hydrogen peroxide. Next time I'm at the speciality grow store where I live, I'm going to ask the sales person what potting mix they use in the store. There is the biggest, healthest, beautiful monstera I've ever seen and their other plants are also in good shape.
